The present invention relates to nuclear magnetic resonance (NMR) imaging and spectroscopy and, more particularly, to a novel class of pulses for refocussing or inversion of spins substantially only in a two-dimensional spatially-selective area, responsive to a single radio-frequency (RF) magnetic field pulse, applied in the presence of an amplitude-modulated magnetic field gradient which reorients itself through the desired two dimensions while that RF pulse is present.
A basic and critical part of a substantial number of NMR imaging and spatially-resolved spectroscopy methods is the selective excitation of nuclear spins in a single spatial dimension. The single dimension selective excitation function is typically carried out by application of a RF magnetic field H.sub.1 signal pulse simultaneous with the presence of a linear magnetic field gradient. Spatial localization can provide slice selection with Gaussian, sinc, sech, and the like function profile shapes, dependent upon the amplitude-modulation or frequency-modulation characteristics of the RF signal pulse. It is difficult to provide, in present selective-excitation applications, complete three-dimensional localization of NMR signals in heterogeneous objects; typically, the NMR signal is spatially localized in only one dimension for each selective RF pulse application, thus necessitating the repeated use of RF and magnetic field gradient pulses, which often require multipulse sequences separated by relatively long periods required for longitudinal spin-magnetization recovery. If, for example, a three-dimensional chemical-shift spectroscopy method uses selective inversion and requires eight different sequence applications (e.g. the eight combinations of three pulse orthogonal magnetic field gradients, with each gradient being turned either on or off) and subsequent addition/subtraction of signals to yield a spectrum from a single fully-resolved selected volume element (voxel), as described by Ordidge et al. (66 J. Magn. Reson. 283 (1986)) and known as the "ISIS" method, it will be seen that erroneous ous signal summation can occur, with concomitant compromise of voxel localization, should the sample position change, relative to the gradient fields during the eight-fold sequence application due to physiological motions during in vivo studies and the like. In conventional NMR imaging procedures, it is often desirable to acquire a high-resolution two-dimensional image restricted to a small portion of a much larger sample volume. Attempting to obtain such an image by conventional methods is likely to produce artifacts caused by NMR response signals derived from excited spins lying outside the region of interest which fold back into the resulting image.
Spatial localization thus remains a major problem in many in vivo spectroscopy and imaging techniques. It is therefore highly desirable to provide a selective excitation pulse capable of spatial localization in more than a single dimension.